1. Field of the Invention
The present invention relates to a heater control method and a heater control apparatus for controlling energization of a heater which is used to activate a detection element of a gas sensor.
2. Description of the Related Art
Conventionally, a gas sensor has been known which includes a detection element including at least one cell composed of a solid electrolyte body and a pair of electrodes and which detects the concentration of a specific gas (e.g., oxygen). The detection element becomes active when its temperature rises, whereby an electromotive force is generated between the pair of electrodes in accordance with a difference in oxygen concentration between two atmospheres separated by the solid electrolyte body. The detection element is heated by the heat of exhaust gas discharged from an internal combustion engine. In addition, in order to activate the detection element quickly, a heater is provided in the gas sensor. A power supply voltage is applied to the heater. However, if the power supply voltage is too high, the temperature rise per unit time becomes large, and an excessive load (mechanical stress) acts on the detection element. As a result, the detection element may crack or suffer from other damage.
An apparatus has been known which overcomes such a drawback by energizing the heater using PWM control (where “PWM” is an abbreviation for “pulse width modulation”) (refer to, for example, Patent Document 1). In the case where an effective voltage applied or more particularly, cumulative power input to the heater is controlled by means of PWM control, a temperature rise curve representing a rise in the temperature of the heater per unit time can be brought closer to a desired temperature rise curve. Thus, a required rate of temperature rise of the heater can be achieved efficiently while reducing the load on the detection element.
[Patent Document 1] Japanese Patent Application Laid-Open (kokai) No. H9-127035.